Hot-dip Al—Zn coated steel sheets that contain 20% to 95% by mass of Al in the coating layer have higher corrosion resistance than hot-dip galvanized steel sheets, as described in Patent Literature 1.
In general, hot-dip Al—Zn coated steel sheets are manufactured by recrystallization annealing and hot-dip coating treatment of a base steel sheet in an annealing furnace on a continuous hot-dip line. The base steel sheet is a thin steel sheet manufactured by hot rolling or cold rolling of a slab. The Al—Zn coating layer includes an alloy phase at an interface between the Al—Zn coating layer and the base steel sheet and an upper layer disposed on the alloy phase. The upper layer includes one portion that mainly contains supersaturated Zn and in which Al is dendritically solidified and another portion between the dendrites. The dendritic solidification portion has a layered structure in the thickness direction of the coating layer. Such a characteristic layer structure makes a corrosion evolutionary path from the surface more complex and makes it difficult for corrosion to reach the base steel sheet. Thus, hot-dip Al—Zn coated steel sheets have higher corrosion resistance than hot-dip galvanized steel sheets that include a coating layer having the same thickness.
There is a growing demand for such corrosion-resistant hot-dip Al—Zn coated steel sheets particularly in the field of construction materials, such as those for roofs and walls, which are exposed to the outdoors for a long period of time, and such steel sheets have also recently been used in the automotive field. However, use of hot-dip Al—Zn coated steel sheets in the automotive field has the following problems.
In the automotive field, it is required to improve mileage by reducing the weight of automobile bodies to decrease CO2 emissions as part of measures against global warming. Thus, there is a strong demand for weight reduction by the use of high-strength steel sheets and gauge reduction by improving the corrosion resistance of steel sheets. However, hot-dip Al—Zn coating treatment of a high-strength steel sheet that contains a large amount of an oxidizable solid-solution strengthening element, such as Si or Mn, results in the formation of an uncoated portion, that is, poor coatability, which results in poor coating appearance. This results from the fact that the reducing atmosphere for reducing Fe in an annealing furnace becomes an oxidizing atmosphere for an oxidizable solid-solution strengthening element, such as Si or Mn, in a steel sheet. More specifically, an oxidizable element Si or Mn undergoes selective surface oxidation (hereinafter referred to as surface enrichment) on the surface of a steel sheet in an annealing process, thereby markedly lowering the wettability of the steel sheet to molten metal.
Patent Literature 2 discloses a technique for improving wettability by adjusting the dew point in a reduction furnace to −10° C. or less. However, this technique cannot reduce the formation of internal oxide.
In general, when used in the automotive field, hot-dip coated steel sheets are supplied to automobile manufacturers after coating with continuous hot-dip coating equipment. The hot-dip coated steel sheets are processed and joined into the shapes of automotive body components and are then subjected to chemical conversion treatment and electrodeposition coating. Thus, when used in the automotive field, the joined portions inevitably include a joint at which steel sheets overlap each other. The joint cannot be easily subjected to chemical conversion treatment or electrodeposition coating and therefore has lower perforation corrosion resistance than portions appropriately subjected to chemical conversion treatment and electrodeposition coating. Thus, there is a problem that the joint has low corrosion resistance.